# C Program to Check if a given Number is Prime or not

### C Program to Check if a given Number is Prime or not

#### Problem Description

This C programming example is to check whether an entered number is prime or not. A prime number is a positive integer which is divisible only by 1 and itself. For example, 3,5,7,11,17 are few prime numbers. This program check for the prime number using the modulus(%) operator.

##### C Program to Check if a given Number is Prime or not - Source code

/* C program to check whether a given number is prime or not */ #include <stdio.h> int main() { int n, i, flag; printf("Enter a number \n"); scanf("%d", &n); if (n <= 1) { printf("Please enter a positive number.", n); return 0; } flag = 0; for (i = 2; i <= n / 2; i++) { if ((n % i) == 0) { flag = 1; break; } } if (flag == 0) { printf("%d is a prime number \n", n); } else { printf("%d is not a prime number \n", n); } return 0; }

#### Program Output

Case 1: Enter a number 451 451 is not a prime number Case 2: Enter a number 79 79 is a prime number Case 3: Enter a number -12 Please enter a positive number.

#### Program Explanation

1. The program accept a positive integer using the scanf function. If the number entered by the user is negative, it prompt the user to enter a positive number. 2. To check for the prime number, modulus operator is used which returns the remainder after dividing the entered number with each number up to n/2 (starting from 2). 3. If the remainder comes out zero in any case, that means the number is not prime and the variable flag is set to 1 and for loop is existed. Otherwise the value of flag remains 0. 4. Depending on the value of variable flag, message is printed on the screen using if-else statement.